1. Field of the Invention
The present invention relates to a broadband wireless access system, and more particularly, to a method for detecting coverage loss during a sleep mode of a mobile station and an apparatus for performing the same.
2. Discussion of the Related Art
It is important that a mobile station with a power source dependent upon a battery for mobility reduces power consumption. In this respect, various methods for reducing power consumption have been suggested. One of the methods is a sleep mode.
The sleep mode means that an operation of a mobile station having no traffic for transmission and reception with a base station is stopped to reduce power consumption.
If the mobile station which is performing communication with the base station in a normal mode or active mode has no traffic for transmission and reception with the base station any more, it transmits a sleep request (MOB_SLP-REQ or AAI_SLP-REQ) message to the base station to enter a sleep mode. The base station that has received the sleep request message transmits a sleep response (MOB_SLP-RSP or AAI_SLP-RSP) message to the mobile station, wherein the sleep response message includes parameters related to the sleep mode of the mobile station, such as initial sleep cycle and a listening window (LW).
The mobile station that has received the sleep response message enters the sleep mode by using the parameters related to the sleep mode.
The mobile station alternately repeats a sleep interval and a listening interval. The mobile station can transmit and receive data to and from the base station for the listening interval. Since the mobile station cannot transmit and receive data to and from the base station for the sleep interval, the data transmitted to the base station for the sleep interval are buffered.
The aforementioned sleep mode operation will be described in more detail with reference to FIG. 1.
FIG. 1 is a diagram illustrating an example of a sleep mode operation procedure of a mobile station in a general IEEE 802.16 system.
Referring to FIG. 1, after the mobile station shifts from a normal mode to a sleep mode, its first sleep cycle includes a sleep window only. Each of a second sleep cycle to the last sleep cycle of the mobile station includes a sleep window and a listening window. The first sleep cycle has a length of an initial sleep cycle included in a sleep response message.
The mobile station receives a traffic indication (TRF-IND) message from the base station for the listening window, wherein the traffic indication message indicates whether there is traffic for transmission to the mobile station in the base station. If the traffic indication message includes negative indication, it means that there is no traffic for transmission to the mobile station in the base station. If the traffic indication message includes positive indication, it means that there is traffic for transmission to the mobile station in the base station.
In FIG. 1, if the traffic indication message, which includes negative indication, is received for the listening window of the second sleep cycle, the mobile station determines whether there is no traffic for transmission to a downlink, and increases the sleep cycle as much as twice more than a previous sleep cycle.
If the mobile station receives the traffic indication message, it increases the sleep cycle in accordance with the following Equation 1.Current sleep cycle=min(2*previous sleep cycle,final sleep cycle)  [Equation 1]
Referring to the Equation 1, the mobile station sets the current sleep cycle to a smaller value of twice of the previous sleep cycle and the last sleep cycle included in the sleep response message. In other words, the mobile station cannot set the sleep cycle to be greater than the last sleep cycle. This is to prevent the sleep cycle from being continuously increased.
If the mobile station receives the traffic indication message, which includes positive indication, for the listening window of the third sleep cycle, it resets the sleep cycle to the initial sleep cycle.
Afterwards, the mobile station can transmit and receive traffic to and from the base station for the listening window included in the MOB_SLP-RSP message. At this time, if the mobile station fails to end transmission and reception of traffic to and from the base station for the listening window included in the sleep response message, it continues to transmit and receive traffic to and from the base station by extending the listening window. If the mobile station which is transmitting and receiving traffic to and from the base station for the extended listening window receives the last PDU indicator or a listening window end flag from the base station, it stops extension of the listening window and enters the sleep window.
In the IEEE 802.16m system, the MOB_SLP-REQ/RSP messages may be replaced with AAI_SLP-REQ/RSP messages, respectively. Also, the mobile station is allocated with a sleep cycle identifier (SCID) suitable for its traffic properties through AAI_SLP-REQ/RSP message exchange when entering the sleep mode. The SCID is an identifier for the mobile station of the sleep mode, and is mapped into parameters (initial sleep cycle, listening window length, TIMF flag, LWEF flag, final sleep cycle, NSCF, T_AMS timer, etc.) used after the mobile station enters the sleep mode. Also, the mobile station and the base station can perform SCID switching or change the parameter values mapped into the SCID for the sleep mode by considering traffic properties of the mobile station.
In the mean time, uplink data may not be transmitted from a mobile station for a certain time period or feedback (ACK or NACK) of downlink data may not be transmitted to a base station. At this time, the base station can facilitate network re-entry of the mobile station by triggering a resource retain timer and storing connection information (context) of the mobile station. This method is provided implicitly in a general mobile communication system. Since the timing and basis that the base station determines that the mobile station has lost network coverage are not definite, the base station should unnecessarily store the connection information of the mobile station.
In this respect, in a general IEEE 802.16m system, a deregistration with context retention (DCR) mode and a coverage loss recovery mode are defined due to a problem that may occur due to non-synchronization between a serving base station and a mobile station if the mobile station loses service coverage of the serving base station.
However, since the mobile station which is being operated in the sleep mode performs normal data transmission and reception for the listening window only, an operation procedure of determining coverage loss and an operation procedure how to perform network re-entry if coverage loss occurs are not defined.